This invention relates to the gasification of carbonaceous materials such as oils, petroleum residua, coals and the like, and is particularly concerned with catalytic gasification operations carried out in the presence of alkali metal-containing catalysts.
It has long been recognized that certain alkali metal compounds can be employed to catalyst the gasification of carbonaceous material such as coal and other carbonaceous solids. Studies have shown that potassium carbonate, sodium carbonate, cesium carbonate and lithium carbonate will substantially accelerate the rate at which steam, hydrogen, carbon dioxide, oxygen and the like react with bituminous coal, subbituminous coal, lignite, petroleum coke, organic waste materials and similar carbonaceous solids to form methane, carbon monoxide, hydrogen, carbon dioxide and other gaseous products. Other alkali metal salts such as alkali metal chlorides, however, have a low catalytic activity when compared to that of the corresponding carbonate and will only accelerate the gasification reactions at a fraction of the rate obtainable with the alkali metal carbonates. It is known that of the alkali metal carbonates, cesium carbonate is the most effective gasification catalyst, followed by potassium carbonate, sodium carbonate and lithium carbonate, in that order. Because of the relatively high cost of cesium carbonate and the low effectiveness of lithium carbonate, most of the experimental work in this area which has been carried out in the past has been directed toward the use of potassium and sodium carbonate. The catalytic activity of sodium carbonate, however, is substantially lower than that of potassium carbonate, therefore attention has been focused in the past on the use of potassium carbonate as a gasification catalyst.
In addition to utilizing individual alkali metal salts as a catalyst for the gasification of a carbonaceous material, it has been proposed to utilize mixtures of alkali metal salts. Specific combinations of alkali metal salts that have been proposed include cesium carbonate and potassium carbonate, cesium carbonate and lithium carbonate, cesium carbonate and cesium chloride, potassium carbonate and lithium carbonate, and potassium carbonate and potassium chloride. When such mixtures of alkali metal salts are used to promote the gasification of a carbonaceous feed material, it is expected that the mixture will accelerate the gasification reactions less than if an equivalent amount of the more active alkali metal compound is used alone and more than if an equivalent amount of the less active alkali metal salt is employed. In a recent publication concerning the use of catalysts in coal gasification it was concluded that there is a substantial need for additional research in general areas related to the use of catalysts in coal gasification. Specifically, it was suggested that a study of catalyst combinations would be a promising area for future research.
In gasification processes using alkali metal-containing catalysts, the cost of the catalyst is a significant factor in determining the overall cost of the product gas. Potassium carbonate is relatively expensive, costing approximately $12.77 per pound mole of potassium. Thus, when potassium carbonate is utilized as a catalyst it is essential that the potassium constituents in the spent solids produced during gasification of the carbonaceous feed material be recovered and reused in the process in order to maintain catalyst cost at a reasonable level. When these potassium constituents are removed from the spent solids exiting the gasifier by water leaching, it has been found that only a portion of the potassium carbonate is recovered and that substantial quantities of makeup alkali metal compounds are therefore required. This adds appreciably to the cost of the gasification operation. In order to decrease the amount of alkali metal makeup compounds necessary, it has been suggested to further treat the char from the gasifier to recover water-insoluble alkali metal constituents by more sophisticated and expensive recovery processes.
The costs of other alkali metal compounds such as potassium chloride ($1.49 per pound mole of potassium), potassium sulfate ($3.29 per pound mole of potassium), sodium carbonate ($1.25 per pound mole of sodium), sodium chloride ($0.79 per pound mole of sodium) and sodium sulfate ($1.95 per pound mole of sodium) are substantially cheaper than potassium carbonate but these compounds have now been found to exhibit only a fraction of the catalytic activity exhibited by potassium carbonate. It would be highly desirable if the compounds mentioned above and other more abundant, less expensive potassium and sodium compounds could be effectively used as gasification catalysts thereby substantially decreasing the initial investment required in the catalyst and obviating the need for expensive secondary recovery techniques to decrease the amount of makeup alkali compounds that would otherwise be required to maintain the catalyst inventory at the required level.